Electrochemical capacitor with a double electric layer

ABSTRACT

The invention relates to elecrical engineering and can be used for producing electrochemical double-layer capacitors having high specific energy and power characteristics and which can store and give off energy at high speed. The essence of said invention lies in the fact that the active mass of a negative polarised electrode ( 2 ) is an organic electroconducting polymer or composite which is based on a carbon or polymer material. A separator ( 3 ) is provide with pores which enables additional oxygen molecules to penetrate. The negative polarized electrode ( 2 ) is made of a polyaniline composite and an activated carbon material or of an activated carbon material composite and polypyrrole. Aqueous solutions of non-organic acids or mixture or salts thereof or thixotropic mixtures of acids and salts or solid proton conductive compositions are used as electrolytes.

FIELD OF THE INVENTION

The invention relates to the field of electrical engineering and can beused in manufacturing electrochemical capacitors having a doubleelectric layer. The electrochemical capacitors of the present inventionhave high specific energy and power characteristics. The capacitors areable to store and release the electric power at a high rate.

The electrochemical capacitors can be used as:

-   -   an electrical transport power supply;    -   auxiliary excitation devices being a part of hybrid transport        means;    -   a starter for internal combustion engines;    -   a power supply for electronic equipment of various types.

BACKGROUND OF THE INVENTION

At present is known an electrochemical capacitor with a double electriclayer, including liquid electrolyte and electrodes made from variousmaterials with a large specific surface U.S. Pat. No. ¹ 4,697,224, Int.Cl. H 01 G 9/00, 1987).

Also is known an electrochemical capacitor with a double electric layer,including solid electrolyte and electrodes made from various materialswith a large specific surface U.S. Pat. No. ¹ 4,713,734, Int. Cl. H 01 G9/00, 1987).

Good values of specific parameters had been obtained for a capacitor inwhich nickel hydroxide and activated carbon-fiber fabric had been usedas positive and negative electrodes correspondingly (WO 97/07518, Int.Cl. H 01 G 9/05, 1997).

The maximum voltage of this capacitor is 1.4V, the specific capacitanceand energy are, correspondingly, 46 F/sm³ and 45 J/sm³.

The closest analogue to the proposed one by the technical essence is anelectrochemical capacitor with a double electric layer, including ahousing, a positive non-polarizable and negative polarizable electrodesmounted inside it, a porous separator separating them, and anelectrolyte. The active mass of the positive non-polarizable electrodecomprises lead dioxide (PCT/RU 97/00353, Int. Cl. H 01 G 9/00, 1997).

The negative polarizable electrode is made from carbon material. Theoperation voltage range of this capacitor is 0.8 to 2.2V, the specificenergy is 56.2 J/g (270 J/sm³). The thickness of the separator used inthe known design is no more than 150 mm.

The specific energy parameters of this capacitor are the highest incomparison with other known capacitors.

A rapid development of the technology allowed to create essentially newtypes of electrochemical capacitors in which new active masses are usedfor manufacturing electrodes, and dramatically widen the range of theirapplication.

Despite of obtaining good results, the problem of increasing thespecific energy and power characteristics of capacitors and decreasingtheir cost for widespread use remains actual at present.

SUMMARY OF THE INVENTION

Problems being solved by the proposed electrochemical capacitor with adouble electric layer are as follows:

-   -   increasing the energy density;    -   increasing the specific power characteristics;    -   obtaining leak-proofness and absence of necessity in        maintenance;    -   decreasing the cost of electrochemical capacitors.

The technical result in the proposed invention is achieved by creatingthe electrochemical capacitor with a double electric layer, including ahousing, a positive non-polarizable and negative polarizable electrodes,mounted inside it, a porous separator separating them, and anelectrolyte, the active mass of the positive non-polarizable electrodecomprising lead dioxide, in which capacitor, according to the invention,the active mass of the negative polarizable electrode is an organicelectric conductive polymer or a composite made on the base of carbonand organic polymer material, and the separator has pores providing anadditional passing of the oxygen molecules.

The invention is also characterized in that the negative polarizableelectrode is made from polyaniline composite and activated carbonmaterial, or from composite of activated carbon material andpolypyrrole, or from an electric conductive polymer polypyrrole.

The invention is also characterized in that the aqueous solutions ofinorganic acids, or of their mixtures, or of their salts, or thixotropicmixtures of acids and salts, or solid proton-conductive compounds can beused as the electrolyte.

Although various electrolytes can be used in capacitor having saidelectrodes, usage of aqueous solutions of inorganic acids or their saltsis preferred.

The invention is also characterized in that the electrochemicalcapacitor with a double electric layer is made leak-proof.

The negative electrode capacitance is a sum of two parallel processes:

-   -   a) forming of the double electric layer;    -   b) redox reactions.

The redox reactions, as a rule, have much lower rate of progress incomparison with a rate of charge and discharge of the double electriclayer.

It is known that in activated carbon materials, the capacitance of redoxreactions is 3-5 times higher than the double electric layercapacitance.

Therefore, in order to increase energy and power characteristics of thecapacitors it is necessary: a) to increase the specific capacitance ofthe negative electrode; b) to increase the contribution of doubleelectric layer capacitance into the whole capacitance of the negativeelectrode; c) to increase the rate of redox reactions.

In this invention said conditions are met due to utilizing variouscomposites on the base of organic compounds and carbon material.

DESCRIPTION OF THE DRAWINGS

The essence of the proposed electrochemical capacitor with a doubleelectric layer is explained by the following description of a structureof electrophysical electrode processes as well as by particularembodiments and drawings, in which:

FIG. 1 shows a cross-section of the electrochemical capacitor with adouble electric layer;

FIG. 2 shows the view A of FIG. 1;

FIG. 3 shows the dependence of voltage on the capacitor (U) andpotentials of positive (φ₊) and negative (φ⁻) electrodes relative to theelectrode Hg-HgSO₄ from the discharge time at the charge current equalto 5 A;

FIG. 4 shows the dependence of voltage on the capacitor (U) andpotentials of positive (φ₊) and negative (φ⁻) electrodes relative to theelectrode Hg-HgSO₄ from the discharge time at the charge current equalto 25 A.

PREFERRED EMBODIMENTS

An electrochemical capacitor with a double electric layer comprises of apositive non-polarizable electrode (1), a negative polarizable electrode(2), a separator (3), a current collector (4) of the polarizableelectrode. The electrode unit is impregnated with necessary amount ofelectrolyte (not shown) and is placed into a housing (5) with a hermeticsealing of current leads (6). The capacitor is provided with anemergency valve (7).

An active mass of the negative polarizable electrode (2) comprises acomposite including carbon or organic polymer material.

The composite materials, in contrast to carbon materials, have thecapacitance of the double electric layer significantly greater than thecapacitance of redox processes, and this leads to a substantial increaseof specific power characteristics of the proposed capacitor.

When charging and discharging, in the negative electrode the followingprocesses proceed:H⁺/e+H[S]⇄2H⁺+[S]+2e,  (1)where H⁺/e is the double electric layer which is formed from the protons(H⁺) interacting by electrostatic forces with quasi-free electrons beingin near-surface layers of the developed surface of the negativeelectrode; H[S] is redox reactions with the participation of weaklybounded or quasi-free hydrogen atom.

In the positive electrode (3), when using an aqueous solution of thesulfuric acid an the electrolyte, the following reaction proceeds:PbO₂+4H⁺+SO₄ ²⁻+2e⇄PbSO₄+2H₂O.  (2)

From formulae (1) and (2) it follows that free charge carriers in thepositive electrode appear as a result of a phase transition of thesecond kind, and in the negative electrode they are in free or weaklybounded state.

Since the nature of an electrical charge origin in the positive andnegative electrodes is different, then contrary to classical capacitorsin which electrodes the electrical charge is in free state, the proposedcapacitor is heterogeneous.

In the proposed electrochemical capacitor with a double electric layerthe best results have been obtained when using the aqueous solution ofthe sulfuric acid having density 1.27 g/sm³ as the electrolyte.

The negative electrodes (2) were manufactured from two-componentcomposite materials of the type A_(x) B_(1−x) (where A and B arecomponent symbols, x is the mass of the component A relative to thewhole composite mass, and 1−x is the mass of the component B relative tothe whole composite mass), on the base of activated carbon material(mainly in the form of carbon-fiber fabric), polyaniline, phenol,hydroquinone and polypyrrole, the value of x being changed from 0 to 1.

The obtained composite materials were subjected to polymerization bymeans of electrochemical treating in the concentrated sulfuric acid.

When using the aqueous solution of the sulfuric acid as the electrolytethe type of double electric layer of the negative electrode in theelectrochemical capacitor (being heterogeneous) is changed during theprocess of charging and discharging. A potential of the positiveelectrode (1) of the charged capacitor is 1.7V relative to the hydrogenelectrode potential, and a potential of the negative electrode (2) isminus 0.5V.

The double electric layer of the negative electrode (2) consists ofprotons placed on the boundary of electrolyte-negative electrodeseparation point, and of free electrons located in near-surface layersof the developed surface.

The voltage of an open circuit (VOC) of the charged capacitor is:U_(VOC)=φ⁺−φ⁻=1,7 V−(−0,5V)=2,2V.

When discharging, free electrons of the double electric layer of thenegative electrode (2) recombine with the positive charge of the PbO₂electrode. This leads to an increase of the negative electrode potentialand transportation of released proton into the positive electrode.

This process proceeds until the potential of the negative electrode (2)reaches the value +0.4V. After this value of the potential the doubleelectric layer, caused by protons and electrons, fully disappears, and anew double electric layer being created by ions HSO₄ ⁻ and free holes innear-surface layers of the negative electrode (2) is formed. Thisprocess proceeds up to the end of discharge.

This process, as a whole, is characterized by the following formula:H⁺/e+HSO₄ ⁻⇄H⁺+HSO₄ ⁻/p+2e,  (3)where p is the hole charge.

The advantage of the proposed capacitor is the great ability forrecombining the hydrogen in the negative electrode (2), thereby itallows to make the capacitor fully leak-proof and maintenance-free.

At the end of charging or when recharging the capacitor a release ofhydrogen from the positive electrode takes place, and hydrogen is notpractically released from the negative electrode.

After transition of hydrogen molecules into the porous space of thenegative electrode the recombination of hydrogen and protons of thedouble electric layer takes place with creating water, i.e., a hydrogencycle is performed.

The hydrogen recombination rate in the negative electrode is ratherhigh, and its value depends on hydrogen pressure in the capacitorvolume. When increasing the hydrogen pressure the rate of hydrogenrecombination raises significantly, thereby allowing to produce a fullcharging of the leak-proof capacitor in 15-20 minutes.

In order to increase the recombination rate of hydrogen being releasedin the positive electrode (1) when charging, the porous separator (3) isused, which separator, in addition to ions, passes the hydrogenmolecules rather effectively.

In the case of the full charging of the capacitor, the redundantpressure of gases inside the housing (1) of the capacitor does notexceed 60-70 kPa, and after finishing the charge the redundant pressuresubstantially fully disappear during 30-40 minutes.

In the case of a continuous cycling of the present capacitor (chargingis performed during 15 minutes, and discharging—30 minutes) theredundant pressure in the volume does not exceed 70 kPa.

In order to increase the hydrogen transition rate in the negativeelectrode (2) the electrolyte amount in the capacitor is normalized sothat the significant part of large pores of the negative electrode (2),which contribution in the process of forming the electrical capacitanceis small, remains not filled with the electrolyte and promotes the rapidtransition of hydrogen.

EXAMPLE 1

The capacitor has been made according to the structural diagram shown inFIG. 1.

An electrode made from material containing lead dioxide (PbO₂) with mass110 g and geometrical dimensions 140×80×1.6 mm³ was used as the positive(non-polarizable) electrode (1) in the capacitor.

A composite material (Pa_(x)Cf_(1−x)) from polyaniline and activatedcarbon fiber with whole mass 18 g and geometrical dimensions 140×80×1.2mm³ was used as the negative (polarizable) electrode (2). The content ofpolyaniline in the negative electrode (2) was 10%. The specificelectrical capacitance of the negative electrode (2) had a value 1200F/g.

The negative electrode (2) with current taps (4) from lead alloy withmass 13 g and geometrical dimensions 140×80×0.1 mm³ comprises of twoelectrically connected parts. The negative electrode (2) (two its parts)is pressed to both surfaces of the positive electrode (1) which isplaced in a pack of separator (3) having thickness 0.08 mm.

An aqueous solution of the sulfuric acid with density 1.27 g/sm³ wasused as the electrolyte. The electrolyte volume was 25 sm³.

The electrode unit was placed into a housing (5) with sealed currentleads (6).

The capacitor is provided with an emergency valve (7) which operates,i.e., releases gases from the inner volume to atmosphere in the casewhen the redundant pressure of gases exceeds the permissible value.

The voltage of the fully charged capacitor was equal to 2.21V. Whendischarging it by the 5 A direct current up to a voltage value on thecapacitor equal to 0.8V, the specific yielded energy (without takinginto account the housing mass) was 216 J/g (911 J/sm³).

The mass and volume of the capacitor are, correspondingly, 190 g and 45sm³.

A change of the content of polyaniline in the active mass of thenegative electrode (2) showed, that when increasing X from 0 to0.1-0.15, the specific capacitance increased, and then graduallydecreased, and when X was equal to 0.9, the specific energycharacteristics decreased 1.3-1.4 times relative to the maximum value.

Therefore, in order to obtain the maximum specific capacitance, theoptimal content of polyaniline in the composite is 10-15%.

The internal ohmic resistance of this capacitor in the beginning and atthe end of discharging was equal to 8.2-10⁻³ Ohm, and at the voltage1.45V on the capacitor decreased to 7.2-10⁻³ Ohm.

When discharging by direct current the potential of the negativeelectrode is changing substantially linearly (FIG. 3).

However, when changing X to a value lower or greater than 0.1-0.15, morerapid fall of negative electrode potential is observed at the end ofdischarging.

As it could be seen from this example, the specific energy of theclaimed capacitor surpasses 3.8 times (by mass) and 3.37 times (byvolume) the corresponding values of the closest analogue.

EXAMPLE 2

A capacitor had been manufactured with the geometrical parameters notedin Example 1. The negative electrode (2) was manufactured by means ofintroducing polypyrrole into the carbon-fiber fabric (Pp_(x)Cf_(1−x))with subsequent electrochemical polymerization.

The polypyrrole content in the negative electrode (2) was 18%. The massof the composite negative electrode (2) was 21 g. The specificcapacitance of the negative electrode was 1050 F/g. The aqueous solutionof the sulfuric acid with density 1.27 g/sm³ was used as theelectrolyte.

The voltage VOC after the full charging of the present capacitor wasequal to 2.09V. When discharging it by the 5 A direct current up to thevoltage 0.8V, the charged capacitor yields 35,2 kj of energy. The massand volume (without taking into account the housing mass) were equal,correspondingly, to 195 g and 46 sm³.

The internal ohmic resistance of this element changes slightly from thebeginning to the end of discharging and, on the average, was 9.3-10⁻³Ohm.

Changing the polypyrrole mass in the negative electrode (2) from 0 to80% showed the following:

-   -   when increasing X from 0 to 0.2 the specific capacitance of the        negative electrode raises from 620 F/g to 1050 F/g;    -   in subsequent increasing the polypyrrole mass the capacitance        decreases to 920 F/g.

It should be noted that in order to obtain the maximum specificcapacitance the optimal polypyrrole content in the composite is 20%.

EXAMPLE 3

In order to obtain a large discharge power the electrochemical capacitorwith thin positive (1) and negative (2) electrodes had beenmanufactured.

The positive electrode (1) with the mass 17 g had geometrical parameters140×80×0.4 mm³. The active mass of the negative electrode (2) with themass 4.7 g comprised of two parts having dimensions 140×80×0.3 mm³, andit was manufactured by introducing 0.45 g of polyaniline into the matrixof carbon fiber with subsequent polymerization. The current tap (4) ofthe negative electrode had dimensions 140×80×0.1 mm³.

The process of assembling the capacitor was performed similar to onedescribed in Example 1.

The capacitor mass (without taking into account the housing mass) was 65g. An aqueous solution of the sulfuric acid with density 1.27 g/sm³ wasused as electrolyte (6).

When discharging with the direct currents of 25 A, 60 A. and 100 A tothe voltage of 0.8V the present capacitor yields, correspondingly, 6 kJ,4.2 kJ, and 3.1 kJ of energy.

The average specific power when discharging with the 100 A directcurrent is 1.90 W/g. The internal ohmic resistance of the capacitor inthe beginning and at the end of discharging was practically similar andequal to 5.2-10⁻³ Ohm.

The voltage on the capacitor and the potentials of the positive (1) andnegative (2) electrodes when discharging with direct current lower than25 A has substantially linear dependence on the discharge time (FIG. 4).

Further increasing the discharge current led to disruption of the lineardependence of the negative electrode (2) potential and, hence, of thevoltage on the capacitor.

This is substantially related with participation of redox reactions inthe discharging process, a rate of which reactions is lower than therate of discharging the double electric layer, and strongly occurs inthe capacitors in which the active mass of the negative electrodeconsists only from carbon material.

In Example 3 it is clearly seen that the proposed capacitor is possibleto provide a high discharge power, and its value will be substantiallyhigher when improving the technology for manufacturing the electrodes.

EXAMPLE 4

The electrochemical capacitor having the negative electrode (2) fromelectric conductive polymer polypyrrole had been manufactured.

Preliminarily, a polypyrrole film is subjected to a long electrochemicaltreating in concentrated sulfuric acid. After washing and drying theelectrode with geometrical dimensions 140×80×0.4 mm³ and mass 3.56 g hadbeen made.

The positive electrode (1) (having active mass PbO₂) had mass 18 g anddimensions 140×80×0.4 mm³.

The process of assembling the capacitor was performed similarly toExample 1 (FIG. 1), and its mass was 69 g (without taking into accountthe housing mass).

The electrolyte was used which was the aqueous solution of the sulfuricacid with density 1.27 g/sm³. Voltage on the fully charged capacitor wasequal to 1,98V.

When discharging by the 0.5 A direct current up to a voltage on thecapacitor equal to 0.8V, the electrical capacitance and yielded energyhave values of, correspondingly, 4.6 kF and 7.41 KJ.

The internal ohmic resistance of this capacitor is 2.4 times higher thanthe internal ohmic resistance of the capacitor described in Example 3.

When increasing the power of discharging, a monotone decrease of yieldedenergy occurs, and when discharging with the 50 A current (the averagespecific power of the discharge is 0.78 W/g) it is equal to 2.31 kJ.

When discharge currents are higher than 12 A, a deviation from thelinear dependence of a negative electrode (2) potential and, naturally,voltage on the capacitor from the discharge time occurs, and as thedischarge current increases, the deviation value also increases.

It is no doubt that when improving the technology of manufacturing thenegative electrodes, it will work out: to decrease the internal ohmicresistance; to increase the specific energy and power characteristics;to widen the working range of voltages.

INDUSTRIAL APPLICABILITY

Thus, aforementioned examples show that when using, in the proposedelectrochemical capacitor with a double electric layer, the compositematerials or electric conductive organic polymers as the active mass ofthe negative electrode taken in the pair with the positive electrodecontaining lead dioxide, its energy and power characteristics surpassesthe corresponding characteristics of the closest analogue.

Apparently, the cost of the stored energy of the proposed capacitor willbe substantially lower than for the closest analogue, since the cost ofcomposite material of the negative electrode does not exceed the cost ofactivated carbon material, and the specific energy of the presentcapacitor is 3.8 times higher.

The claimed capacitor allows to perform both parallel and seriesconnection of elements and to create on its base a capacitor batteriesfor various values of working voltages and capacitance.

The capacitor can have various forms and configurations of electrodesand housing. The disclosed examples only demonstrates severalcharacteristics of the present invention and do not limit itspossibilities. Introducing evident different technological changes intoit will lead to improving the capacitor characteristics.

1. An electrochemical capacitor with a double electric layer, thecapacitor comprising: a positive non-polarizable electrode, a negativepolarizable electrode having pores, the positive and negative electrodesbeing within a housing, a porous separator separating the electrodes,pores of the separator allow passage of oxygen molecules, and anelectrolyte, wherein an active mass of the positive non-polarizableelectrode comprises lead dioxide, an active mass of the negativepolarizable electrode includes an organic electric conductive polymer ora composite material based on a carbon and organic polymer material, anda part of the pores of the negative electrode is not filled with theelectrolyte.
 2. The electrochemical capacitor with a double electriclayer according to claim 1, wherein the negative polarizable electrodeis formed from the composite material based on the carbon and organicpolymer material, wherein the organic polymer material is polyaniline,and the carbon is an activated carbon material.
 3. The electrochemicalcapacitor with a double electric layer according to claim 1, wherein thenegative electrode is formed from the composite material based on thecarbon and organic polymer material, wherein the organic polymermaterial is polypyrrole, and the carbon is an activated carbon material.4. The electrochemical capacitor with a double electric layer accordingto claim 1, wherein the negative electrode is formed from the electricconductive polymer material, wherein the electric conductive polymermaterial is polypyrrole.
 5. The electrochemical capacitor with a doubleelectric layer according to claim 1, wherein the electrolyte contains anaqueous solution selected from the group consisting of inorganic acids,or their mixtures, or salts.
 6. The electrochemical capacitor with adouble electric layer according to claim 1, wherein the electrolytecontains thixotropic mixtures of acids or salts aqueous solution.
 7. Theelectrochemical capacitor with a double electric layer according toclaims 1, wherein the electrolyte contains solid proton-conductivecompounds.